1. Field of the Invention
The present invention relates to a semiconductor device having an active layer of a semiconductor thin film formed on a substrate having an insulating surface, and particularly to a thin film transistor in which an active layer is made of a crystalline silicon film.
2. Description of the Related Art
In recent years, attention has been paid to a technique in which a thin film transistor (TFT) is constituted by using a semiconductor thin film (thickness of about hundreds to thousands A) formed on a substrate having an insulating surface. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device, and especially, its development as a switching element for an image display device has been hurried.
For example, in a liquid crystal display device, attempts have been made to apply TFTs to any electric circuit such as a pixel matrix circuit for respectively controlling pixel regions arranged in a matrix form, a drive circuit for controlling the pixel matrix circuit, and a logic circuit (processor circuit, memory circuit, etc.) for processing data signals from the outside.
In the present circumstances, although a TFT using an amorphous silicon film as an active layer is put into practical use, an electric circuit required to have further high speed operational performance, such as a drive circuit and a logic circuit, demands a TFT using a crystalline silicon film (polysilicon film).
As a method of forming a crystalline silicon film on a substrate, techniques disclosed in Japanese Patent Unexamined Publication No. Hei 6-232059 and No. Hei. 6-244103 by the present applicant are well known. The techniques disclosed in these-publications enable the formation of a crystalline silicon film having excellent crystallinity by using a metal element (especially nickel) for promoting crystallization of silicon and by a heat treatment at 500 to 600° C. for about four hours.
Japanese Patent Unexamined Publication No. Hei. 7-321339 discloses a technique for carrying out crystal growth substantially parallel to a substrate by utilizing the above techniques. The present inventors refer to the formed crystallized region as especially a side growth region (or lateral growth region).
However, even if a drive circuit is constituted by using such a TFT, the drive circuit is still far from the state of completely satisfying the required performance. In the present circumstances, especially it is impossible to constitute a high speed logic circuit requiring electric characteristics of extremely high performance to realize both high speed operation and high withstand voltage characteristics at the same time, by a conventional TFT.
As described above, in order to attain the higher performance of an electro-optical device and the like, it is necessary to realize a TFT having performance comparable wish a MOSFET formed by using a single crystal silicon wafer.